JPS62199722A - Improvement of residual stress of cylindrical body - Google Patents

Abstract

PURPOSE:To easily improve the residual stress of a cylindrical body by heating the cylindrical body from the outside surface side for a short period to generate the distribution of the temp. difference between the inside and outside surfaces of the cylindrical body in the longitudinal direction thereof and the distribution of the average temp. in the thickness of the cylindrical body in the longitudinal direction thereof. CONSTITUTION:The cylindrical body having the residual tensile stress on the inside surface by welding, etc., is heated for the short period from the outside surface side. The distribution of the temp. difference between the inside and outside surfaces of the cylindrical body in the longitudinal direction thereof and the distribution of the average temp. in the thickness of the cylindrical body in the longitudinal direction thereof are thereby generated. The tensile stress on the inside surface of the cylindrical body occurring in the above-mentioned former distribution and the tensile stress on the inside surface of the cylindrical body occurring in the latter distribution are superposed and the above-mentioned heating is so executed that the tensile stress on the inside surface of the cylindrical body, i.e., the sum thereof exceeds the yield point in the range of the above-mentioned residual stress to be improved. The heating is thereafter stopped and the above-mentioned residual tensile stress is modified to the extremely small residual tensile stress or residual compressive stress, by which the residual stress of the cylindrical body is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for improving residual stress in the inner surface of a cylindrical body, particularly a welded cylindrical body, including a welded portion.

[Conventional technology]

Tensile stress often remains on the inner surface of a welded cylinder at and near the weld, and this residual tensile stress causes stress corrosion cracking and the like. Therefore, it is necessary to modify the residual tensile stress to an extremely small value, or rather to a residual compressive stress (this is referred to as improving residual stress). The residual stress on the inner surface of the cylinder can be improved by applying tensile stress and compressive stress exceeding the yield point to the inner and outer surfaces K of the cylinder, respectively. Residual stresses on the inner surface of the body are converted into extremely small tensile stresses or into compressive stresses.

Conventionally, the method of improving the residual stress on the inner surface of a welded cylinder is to heat the cylinder from the outside and at the same time cool the inner surface by forcing a cooling fluid to flow inside, thereby bringing the temperature below the transformation point. A method has been used in which a temperature difference is generated between the inner and outer surfaces of the cylindrical body, and this temperature difference is used to apply stress exceeding the yield point as described above. However, with this method, if the target cylinder is a double-cylindrical outer cylinder and the inner surface of the nuclear outer cylinder cannot be forcibly cooled with cooling fluid, the temperature difference between the inner and outer surfaces of the outer cylinder cannot be made sufficiently large. Therefore, residual stress cannot be improved.

As a heat treatment method for improving the residual stress of a double cylinder in consideration of this point, JP-A-60-141825 describes a cylindrical body in which a double cylinder and a single cylinder are connected in the nozzle part of a reactor pressure vessel. something is disclosed. This method involves installing a high-frequency induction heating coil that widely covers the double cylinder part and the double cylinder part, first applying a temperature difference in the thickness direction to the inner and outer surfaces of only the single cylinder part, and then In order to create a temperature difference in the thickness direction of both the inner and outer surfaces of the outer cylinder of the double cylindrical part, heating was performed for a very short period of time, and then only the heating of the double cylindrical part was stopped and a time difference was left. The heating of the single cylinder portion is then stopped. This has the advantage that since the heating time of the double cylindrical portion is extremely short, there is no need to increase the internal temperature of this portion. However, this method requires several steps that involve changing the heating range: single cylinder heating → - heavy cylinder and double cylinder heating (short time heating → double cylinder heating stop, - heavy cylinder heating continued → - heavy cylinder heating stop). Requires steps. In addition, since this is a stress improvement treatment that utilizes only the temperature difference between the inner and outer surfaces of the plate, it is necessary to generate a strain greater than the yield point only by the temperature difference between the inner and outer surfaces. Must have been achieved over a length of time. Furthermore, this method requires adjustment of the consistency of the heating equipment (current-voltage phase adjustment, etc.) because the heating range is changed during the heat treatment, that is, the coils are switched.

Furthermore, Patent No. 957324 also discloses a heat treatment method for improving residual stress that utilizes only the temperature difference between the inner and outer surfaces, but as in the above publication, when the required longitudinal distribution of the temperature difference between the inner and outer surfaces cannot be secured within a certain range. No effective stress improvement is achieved. In addition, in order to bring the outer surface temperature below the transformation temperature and increase the temperature difference, it is necessary to lower the inner surface temperature, but it is difficult to forcefully cool the cooling water in the annular gap of a double cylinder, so natural convection is necessary. Because it does not occur, it immediately enters a boiling and cooling state,
As a result, the inner surface temperature of the outer cylinder becomes high, making it difficult to maintain a temperature difference between the inner and outer surfaces. For this reason, the above-mentioned patent is also not optimal as a stress improvement heat treatment for the outer cylinder of a double cylinder.

[Problem that the invention seeks to solve]

As mentioned above, the conventional technology is a residual stress improvement method that utilizes only the thermal strain caused by the temperature difference between the inner and outer surfaces of the cylinder.
This temperature difference alone must be sufficient to generate a stress above the yield point, and this temperature difference must be realized over a considerable range in the longitudinal direction (
For example, 5US304 with an outer diameter of 350 m and a plate thickness of 305 m.
For stainless steel pipes, a temperature difference of 250° C. between the inside and outside surfaces must be achieved over a length of about 200 m). However, this is not the case with cylinders whose cross-sectional shape is not constant, or cylinders where forced cooling of the inner surface is difficult and only natural convection or boiling cooling of the cooling liquid is possible (for example, a double-walled outer cylinder). is difficult to achieve.

The present invention aims to provide a method that can avoid the above-mentioned problems and improve the residual stress on the inner surface even in the case of such a cylindrical body.

[Means for solving problems/points]

The present invention is a method for improving the residual stress of a cylinder by changing the residual tensile stress on the inner surface of the cylinder into extremely small residual tensile stress or residual compressive stress. This heating produces a longitudinal distribution of the temperature difference between the inner and outer surfaces of the cylinder and a longitudinal distribution of the average temperature within the thickness of the cylinder, and the heating causes the tensile stress on the inner surface of the cylinder due to the former distribution. The system is characterized in that the sum of the tensile stress on the inner surface of the cylinder due to the latter distribution exceeds the yield point over the range to be improved for residual stress on the inner surface of the cylinder, and heating of the hind limbs is stopped. be.

[Effect]

When heating produces a longitudinal distribution of the average temperature within the plate thickness of the cylinder, due to thermal expansion, the parts of the cylinder with a higher average temperature swell in the radial direction as well as the parts with a lower average temperature, which applies stress to the cylinder. . For example, in the case of a distribution where the average in-thickness temperature Ttn is higher by a certain value in a narrow longitudinal range (double hatched range) than in other longitudinal ranges, as shown in Figure 2 (,), At the third point, as shown in FIG. 2(b), tensile stress is applied to point A on the outer surface, compressive stress is applied to point B on the inner surface, and tensile stress is applied to points C1 and C2 on both sides of the inner surface. In addition, as shown in Fig. 3(,), when the thickness average temperature Tm is distributed over a relatively wide range in the longitudinal direction and is higher by a certain value than in other longitudinal directions, the third As shown in Figure 3), compressive stress is applied to point 311B on the inner surface near both ends of the range, and point C3 on the inner surface next to it is
A tensile stress is applied to +04, and a range where no stress is applied occurs between points Bl and 82.

In actual heating, the distribution does not have a rectangular shape as shown in the figure.
Figure (,) shows a distribution with a sharp mountain-shaped peak, and Figure 3 (b) shows a trapezoidal distribution with rounded corners.
Its effect on stress is essentially the same as above.

On the other hand, if heating produces a temperature difference between the inner and outer surfaces of the cylindrical body, such that the outer surface of the cylinder is higher in temperature than the inner surface, as shown in FIG. b
), compressive stress is applied to the outer surface of the cylinder, and tensile stress is applied to the inner surface.

Therefore, if the longitudinal distribution of the average temperature within the plate thickness and the longitudinal distribution of the temperature difference between the inner and outer surfaces are simultaneously generated by heating, the stress caused by the former and the stress caused by the latter will be superimposed. Become. In this case, even if there is a part where the tensile stress on the inner surface of the cylinder due to the former and the tensile stress on the inner surface of the cylinder due to the latter do not exceed the yield point independently, the inner surface of the cylinder which is the sum of their superimposition is The tensile stress can be increased to exceed the yield point over the residual stress improvement target range, and thereby the residual stress on the inner surface of the cylinder after heating is stopped can be changed to an extremely small tensile stress or compressive stress.

〔Example〕

An embodiment of the present invention applied to improve residual stress in a nozzle welded structure of a reactor pressure vessel 9 will be described with reference to FIG.

In Fig. 1, 1 is the inner cylinder (thermal three),
It branches off from the outer cylinder 3' at a branching part 2. The outer cylinder 3' is welded to another outer cylinder 3'', which forms the tip of the nozzle of the reactor pressure vessel 9, at a welding part 4. The inner cylinder 1 and the inner cylinder 1 form a double cylindrical part, and an annular gap is formed therebetween. The outer cylinder 3 and the inner cylinder 1 are made of stainless steel W4. The outer cylinder 3' becomes a -1 cylinder 6 beyond the branching part 2, and is welded to another single cylinder 6' by a welding part 7.

The purpose of the residual stress improvement method of this embodiment is to convert the residual tensile stress on the inner surface of the outer cylinder 3 in the welded portion 4 and its vicinity into residual compressive stress.

8 is an induction heating coil arranged around the outer cylinder 3 in order to carry out the method of the present invention, and the coil 8-1 is connected in series.
．． 8'-2 and 8-3 (the upper half is not shown). When power is applied to the coil 8, the distribution of the average temperature within the thickness of the outer cylinder 3 in the longitudinal direction of the outer cylinder 3 has one peak slightly to the right of the welding part 4, and the other peak near the branch part 2. These coils are arranged so as to produce a peak and a valley between the two peaks. To do this, the right coil 8-1 should be placed to the right of the welding part 4 and at a narrow distance from the outer cylinder 3'', and the left coil 8-3 should be placed outside facing the branch part 2. The middle coil 8-2 is arranged with a narrow distance from the cylinder 3', and a wide distance from the outer cylinder 3', with the intermediate coil 8-2 facing the IC.

Cooling water 5 is filled in the annular gap between the outer cylinder 3 and the inner cylinder 1 of the double cylindrical part. It is difficult to force the cooling water 5 to flow because the annular gap is narrow and it ends at the branch 2.

There is also cooling water 10 inside the cylinder 1 and the single cylinders 6 and 6'.
satisfy. Although the cooling water 10 does not have to be forced to flow, in this embodiment, a case will be described in which it is forced to flow.

Under such arrangement and cooling conditions of the induction heating coils, a large input is simultaneously applied to the induction heating coils 8-1 to 8-3, heating is performed for a short time, and then heating is stopped and cooling is performed to room temperature. Curves A and B in Fig. 5 respectively represent the outer cylinder 3'' at this time.
These are the temperature time curves of the outer and inner surfaces of . Looking at the inner surface temperature B of the outer cylinder 3'', the cooling water 5 in the annular gap is not forced to flow, so there is no natural convection cooling, and since the annular gap is narrow, boiling cooling occurs at point B1. If heating is continued further, film boiling will occur at 82 points. 1. The temperature rise on the inner surface will become steeper.On the other hand, curves C and D are the one-hour temperature curves on the outer and inner surfaces of the outer sleeve, respectively. As a result, the distance between the coil 8-2 and the outer cylinder 3' is increased, and the coil arrangement is such that the temperature rise on the side of the outer cylinder 3' is suppressed to a low level.As a result, the outer surface temperature of the outer cylinder 3' is reduced. The height of the 3" outer cylinder is also lower.

FIG. 6 illustrates the thermal stress shimmy run analysis results of the above heat treatment according to this example. The temperature and temperature difference shown in the figure are those at the end of the heating time, and the residual stress is the residual stress when the temperature returns to room temperature after heating. The horizontal axis represents the distance in the longitudinal direction of the cylinder with the center of the welded portion 4 as the origin. In the figure, Ti is the temperature of the inner surface of the outer cylinder 3, Tm is the average temperature in terms of the plate thickness of the outer cylinder 3, ΔT is the temperature difference between the inner and outer surfaces of the outer cylinder 3, and σt is the inner surface of the outer cylinder 3. represents the residual stress in the longitudinal direction, and σθ represents the residual stress in the circumferential direction on the inner surface of the outer cylinder 3. A positive value of residual stress means tensile residual stress, and a negative value means compressive residual stress.

σ7 in Figure 6. As is clear from the graph of σθ, the residual stress on the inner surface of the outer cylinder 3 after the heat treatment according to this example is:
It can be seen that both the longitudinal residual stress σ6 and the circumferential residual stress σθ are compressive stresses, and that the intended purpose of residual stress improvement has been achieved. This will be addressed below.

The minimum temperature difference ΔTm1n required to generate residual compressive stress on the inner surface of the outer cylinder 3 using only the temperature of the inner and outer surfaces of the outer cylinder 3 is expressed by the following equation.

Here, α is the coefficient of linear expansion of the outer cylinder 3, E is the Young's modulus, and σa is the stress drop. Applying the values for stainless steel to these, it becomes ΔTm1n4250°C. However, the 6th
In the distribution of ΔT in the figure, ΔT exceeds the value of ΔTm1n above only in the part of the outer cylinder on the right side of the welded part 4 and in the part of the branch part 2 between the inner cylinder and the outer cylinder, and in between. ΔT forms a valley where it does not reach the value of ΔTm1n. On the other hand, the plate thickness average temperature Tm is on the higher temperature side than the 21 distribution in the double cylinder part due to the fact that the inner surface of the outer cylinder is heated to a high temperature until film boiling of the cooling water occurs. The distribution has a peak, and the position of the peak and the length of the valley between them are almost the same as that of ΔT.

Such Tm distribution can be schematically represented as shown in FIG. 7 (,), and the double diagonal lined portion in the figure corresponds to the peak portion. The stress on the inner surface of the outer cylinder 3 due only to this Tm distribution becomes tensile stress at the valleys of the Tm distribution, and compressive stress at the peak portion, as shown in FIG. 7(b). Therefore, as mentioned above, at the peak part of the Tm distribution, ΔT
Since ΔTm1n sufficiently exceeds ΔTm1n, as a result, that portion also becomes tensile stress. In this way, the stress on the inner surface of the outer cylinder resulting from the superimposition of the stress due to the Tm distribution and the stress due to the 21 distribution becomes tensile stress in the entire region. All stress becomes compressive stress.

In short, by compensating for the shortfall in ΔT by the effect of Tm distribution, the tensile stress on the inner surface of the outer cylinder due to ΔT during heating and the tensile stress superimposed with that due to Tm distribution exceeds the yield point. Later, residual stress on the inner surface of the outer cylinder can be made into compressive stress in all regions.

FIG. 8 shows another example in which the first peak is set so that only one peak of Tm
TI, ΔT + 'rm 1σ6. when large heat input and short time heating are performed only with coil 8-1.8-2 in the figure. σ
θ (these meanings are the same as in the previous example)
i exceeds 200° C. even though film boiling cooling is used to increase Tm. Therefore, ΔT is on the contrary smaller. Because ΔT is small in the outer cylinder 3'', the compressive residual stress in this part is somewhat small.However, on the inner surface of the outer cylinder 3', both σ6 and σθ are T.
Due to the effect of the peak distribution of m, υ becomes a sufficiently large residual compressive stress.

The above embodiments have been described for the case of a double cylinder, but even if it is a normal single cylinder pipe, if cooling water cannot be forced to flow inside, the temperature difference between the inner and outer surfaces due to short-term heating may be reduced. Residual stress can be improved by the method of the present invention that combines the distribution of plate thickness average temperature.

FIG. 9 shows an embodiment in which the method of the present invention is applied to improve residual stress in a weld 4 between a tube sheet 12 and a body 13 of a heat exchanger. Because there are obstacles such as heat transfer tubes 14 and partition plates 15 inside the fuselage 13, it is not possible to sufficiently increase the cooling efficiency of the inner surface of the fuselage 3 by the cooling water 11, so it is not possible to improve stress by using only the temperature difference in the plate thickness direction. is difficult. Therefore, in this embodiment, the distribution in the longitudinal direction of the body of the temperature difference ΔT in the plate thickness direction is made trapezoidal across this welded part, and the plate thickness average temperature Tm is located on the right side of the welded part 4 at one end of the trapezoidal distribution of the above-mentioned ΔT. The induction heating coils 8-4 and 8-5 are arranged so as to have a peak in the opposite direction, and high heat input and rapid heating is performed for a short time. Coils 8-4 and 8-5 are continuous bright line annular coils, but coil 8-5 is similar to coil 8-5.
Since the coil 8-4 is installed closer to the body 13 than the coil 8-4, the plate thickness average temperature Tm can be higher than that of the portion heated by the coil 8-4. Due to the distribution of Tm, tensile bending stress is generated in the welded part 4 during heating, and the welded part 4 easily undergoes tensile yield due to the tensile stress due to ΔT, and after cooling, the stress is reduced. The residual compressive stress is obtained by inversion. In this embodiment, since the tube sheet 12 has high rigidity, it effectively acts to generate the above-mentioned bending stress in the welded portion 4.

As can be seen in each of the embodiments described above, producing the peak in the longitudinal distribution of the average temperature within the plate at a position slightly distant from the weld zone further brings about the following advantages.

That is, 5US304 stainless steel welds become more susceptible to corrosion when exposed to high temperatures, and carbon steel and the like become brittle.

Therefore, even though the welded portion is the outer surface, it is not preferable that the welded portion be exposed to high temperatures. According to the present invention, the above-mentioned fear is eliminated because the welded portion is not exposed to high temperatures due to the above-mentioned peak position.

〔Effect of the invention〕

According to the present invention, it is difficult to cool the inner surface of the cylinder with forced fluid during heating, and natural convection cooling or boiling cooling is required, such as a double-walled outer cylinder, or a cylinder whose cross-sectional shape changes. As in the case of a cylindrical body, it is not possible to create a sufficient temperature difference between the inside and outside surfaces over the range to be improved by residual stress.
Therefore, even for cylinders whose residual stress cannot be improved only by stress based on the temperature difference between the inner and outer surfaces, it is possible to combine the stress based on the temperature difference between the inner and outer surfaces with the stress based on the longitudinal distribution of the average temperature within the plate thickness. As a result, internal residual stress can be improved, and complicated control such as changing the heating section during heating is not required, simplifying the operation.

Of course, the present invention is not limited to the case of a cylindrical body as described above, but can also be applied to a normal cylindrical body whose inner surface can be forcedly cooled.

[Brief explanation of drawings]

FIG. 1 is a partial cross-sectional view showing an embodiment in which the present invention is applied to a double cylinder part of a reactor pressure vessel nozzle, and FIG. 2(a)
, (b) is a diagram showing an example of the longitudinal distribution of the average temperature within the plate thickness of the cylinder and the stress on the inner surface due to it, and Figure 3 (a)
), (b) is a diagram showing another example of the longitudinal distribution of the average temperature within the thickness of the casing and the resulting stress on the inner surface.
Figures (a) and (b) are diagrams showing an example of the longitudinal distribution of the temperature difference between the inner and outer surfaces of the cylindrical body and the resulting stress on the inner surface, respectively. Figure 5 shows the relationship between time and temperature in an example of the present invention. Figure 6 shows the analysis results of the same example, and Figures 7 (a) and (b) show the longitudinal distribution of the average temperature within the thickness of the outer cylinder plate and the stress on the inner surface due to it in the same example. FIG. 8 is a diagram showing an analysis result of another embodiment of the present invention, and FIG. 9 is a diagram showing an embodiment in which the present invention is applied to a body welded portion of a heat exchanger. 1... Inner cylinder 2... Branch part 3... Outer cylinder 4, 7... Welding part 8... High frequency induction heating coil 12... Tube plate 13... Fuselage L construction Figure 2 Figure 3 Figure 4 Time Figure 6 Longitudinal force PJ distance from the center of contact group 4 (plane O Figure 7

Claims (1)

[Scope of Claims] 1. A method for improving residual stress in a cylindrical body for changing residual tensile stress on the inner surface of the cylindrical body into extremely small residual tensile stress or residual compressive stress, the method comprising: Heating produces a longitudinal distribution of the temperature difference between the inner and outer surfaces of the cylinder and a longitudinal distribution of the average temperature within the thickness of the cylinder, and the heating causes the temperature difference inside the cylinder due to the former distribution. The heating is stopped after the sum of the tensile stress on the surface and the tensile stress on the inner surface of the cylinder due to the latter distribution exceeds the yield point over the range to be improved for residual stress on the inner surface of the cylinder. A method for improving residual stress in a cylindrical body. 2. Both of the above distributions have at least 1
A method for improving residual stress in a cylindrical body according to claim 1, which has two peaks. 3. The method for improving residual stress in a cylindrical body according to claim 1, wherein the former distribution is a trapezoidal distribution, and the latter distribution has a peak at one end of the trapezoidal distribution. 4. Claim 1, wherein the cylinder is a double cylinder outer cylinder.
, the method for improving residual stress in a cylindrical body according to item 2 or 3.